Removal of certain fission product metals from liquid bismuth compositions



REMOVAL OF *CERTAEN FISSION PRODUCT METALS FROM LIQUID BISMUTH COMPOSI-TIONS Orrington E. Dwyer, Wading River, N .Y., and Herbert E. Howe,North Plainiieid, and Edward R. Avrutik, Nixon, N1, assignors to theUnited Sitates at America as represented by the United States AtomicEnergy Commission No Drawing. Application August 20, 1958 Serial No.756,263

7 V 8 Claims. (Cl. 7584.1)

The present invention relates to a process for selectively removingmetals dissolved in a liquid bismuth composition. More particularly itrelates to a method of selectively extracting certain fission productmetals from a liquid nuclear fuel composition in which the nuclear fuelis uranium and the carrier or solvent is liquid bismuth.

Liquid metal fuel reactors in which the fuel is a solution of uranium inliquid bismuth have been proposed as a useful source of power. Theprinciples of construction and operation of a liquid metal fuel reactorare described in Nucleonics, July 1954, volume 12, pages l142, and inthe Proceedings of the First International Conference on the PeacefulUses of Atomic Energy. A typical fuel which can be used for a liquidmetal fuel reactor is a dilute solution of about 500-4000 parts permillion by Weight uranium in molten bismuth for use at a temperature inthe range 400550 C. In addition, up to 350 parts per million (by weightof bismuth) each of zirconium and magnesium are used as corrosion masstransfer inhibitors. As the reactor operates, fission productsaccumulate in the fuel. Some of these fission products are termedreactors poisons since they have large capture cross sections forneutrons and therefore reduce the number of neutrons available forproducing fission.

In order to operate such a reactor economically, it is necessary thatthe irradiated fuel composition be processed periodically to render boththe uranium and the bismuth free of neutron-absorbing fission products.A

convenient and economic process is desirable because the cost of suchprocessing is reflected directly in the cost of power from the reactor.

ing fission products from a liquid metal nuclear fuel composition. Aprincipal object of the invention is to reduce the concentration offission products in a solution of uranium in bismuth. A further objectof the invention is to provide a method for removing certain metalspreferentially from a solution of uranium in bismuth. Another object ofthe invention is to avoid high losses of uranium during decontaminationof a uranium-bismuth nuclear fuel. is to provide a process forrecovering bismuth substantially free of neutron-absorbing fissionproducts. Other objects will become apparent from the description whichfollows. i

The products of fission of a liquid metal fuel reactor in which theliquid metal is bismuth and the fuel is uranium may be classified intothree main groups. All are present after the liquid metal fuelcomposition has been exposed to neutrons to cause fissions. They are:

1) The volatile fission products consisting principally of krypton andxenon with trace amounts of bromine and iodine. In removing fissionproducts from an irradiated nuclear fuel of the type described, thevolatile fission products are generally removed first by passing thefuel composition through a degassing apparatus.

Still another object of the invention Patented Nov. 24, 1959 iQQ (2) Thenon-volatile fission products which form. chlorides more stable than UCIthe most stable chloride of uranium. After the volatile fission productshave been removed, this class of non-volatile fission products isremoved from the fuel, by extracting them with a fused chloride saltmixture which may contain an oxidizing agent. A suitable carrier saltmay be the ternary eutectic, NaCl-KCl-MgCh, and the oxidizing agent maybe BiCl Sufficient oxidant is added to the salt to form chlorides of thefission products which form chlorides more stable than UCl leavingsubstantially all of the uranium and the remaining fission products inthe molten ,bismuth. Processes of this type for. removing these fissionproducts are described in US. Patent No. 2,758,023 to Bareis andco-pending application Serial Number 511,810, filed May 27, 1955, for R.H. Wiswall.

(3) The non-volatile fission products which form chlorides less stablethan UCl These fission products are relatively unaffected by theoxidizing fused salt mixture and remain in the molten bismuth along withthe uranium. The metals in this class include palladium, ruthenium,rhodium, selenium, tellurium, niobium and zirconium. The presentinvention is particularly adapted for the separation of this third groupof fission products from liquid bismuth or liquid bismuth containinguranium dissolved therein. The term fission product or fission productmetals as used hereinafter will refer to this third class of fissionproducts, i.e. those which form chlorides less stable than UCl In thepractice of this invention, we can preferentially extract fissionproducts from liquid bismuth or from a dilute solution of uranium inliquid bismuth. We have found that these fission products can bepreferentially extracted from liquid bismuth by contacting and mixingthe liquid bismuth with very small amounts of zinc at a temperature inthe range 400-500 C. At higher temperatures the zinc has an appreciablevapour pressure and will distill from the mixture. While the zinc may bemixedwith the bismuth solution at temperatures lower than'400" C., itrequires an inconveniently long time before a homogeneous melt isformed. The liquidbismuth-zinc composition is agitated to insure uniformdistribution of the zinc, and may then be cooled toany temperature aboveits freezing point, i.e., at a temperature above that at whichbismuth-zinc eutectic forms (255 C.). Cooling results in theprecipitation of a solid zinc-rich phase containing the fission productelements. The solidified zinc phase is in the form of finely dividedparticles dispersed in the liquid bismuth phase. These zinc-phaseparticles,less dense than bismuth, rise slowly through the liquidbismuth to form a dross which can be skimmed off the surface of theliquid bismuth leaving the bismuth with a substantially reduced fissionproduct content. Another convenient way of separating the solid zincphase from the liquid bismuth is by filtering the melt through a porouscompact of a finely divided inert material such as graphite. :In somecases cooling over a relatively narrow temperature range is suflicientto form a zinc phase containing a high concentration of the fissionproducts from the bismuth.

Before the liquid bismuth, now reduced in its fission product content,can be returned for use in the reactor, it must be treated to remove anyresidual zinc. Since zinc is soluble to the extent of approximately 2.7%by weight in liquid bismuth, there will always be some residual zincwhich must be removed in order to produce a zincfree bismuth product.Several methods for removing zinc from the bismuth are known. One methodwhich can be used is distillation at about 550 C.600" C. under vacuum toremove the excess zinc.

The concentration of zinc used in extracting the fission products fromthe molten bismuth is an important consideration in the eflicientpractice of our invention. In a reactor using 150 tons of bismuth, forexample, it is apparent that only the necessary amount of zinc should beused for removing the fission products. Any excess zinc would representanother contaminant requiring additional processing to purify thebismuth. It is therefore, preferred to use only the minimal amount ofzinc which will effectively remove the fission products from the moltenbismuth phase.

We have found that effective removal of the fission products from thebismuth phase can be accomplished using hypo-eutectic compositions ofzinc, i.e., less than 2.7% by weight of the bismuth. As little as .2% to2.5% zinc by Weight of the bismuth will efiectively remove virtually allthe fission'product content from the liquid bismuth phase. Ourexperiments have shown that separation of the fission products cannot beaccomplished by using less than .2% of zinc by weight of the bismuth,regardless of fission product concentration. The practice of ourinvention will be more fully understood from the following examples inwhich all concentrations are expressed in terms of weight percent ofbismuth. It is not intended to limit the scope of the invention to thedetails of these examples.

EXAMPLE I This example illustrates the degree to which fission productscan be removed from a liquid bismuth solution laden with these products.Two samples of bismuth, each containing fission products atconcentrations as indicated in the first lines of Tables 1A and 1B wereheated to 500 C. in a graphite crucible exposed to the atmosphere. Themolten bismuth was then cooled to 450. C. at which point .5% zinc wasadded to'one and 1.5% Zinc was addedto the other bismuth solution. Themixtures were agitated to insure the formation of a uniform composition.Cooling of these solutions resulted in the formation of a thin film of azinc-rich dross on thesurface of the liquid bismuth melt. Samples weretaken of the liquid bismuth phase at the temperatures indicated inTables 1A and 1B, and it was found that the concentrations of thefission product metals in the'bismuth I decreased with decreasingtemperature. The samples were taken by immersing a closed-end tubethrou'gh'the 'dross formed at the liquid bismuth surface and into thebismuth solution. The immersed end of the tube was then broken againstthe crucible wall and a liquid bismuth sample drawn into the tube. v 5%and 15% zinc, respectively, are summarized in Tables 1A and 1B below.

Table 1A 7 Bismuth With 0.5% Zine Parts Per Million Temperature VRuthenium Palladium Rhodium Tellurium (No Zn 500? o.) 44 26 12 100 450C.(Zn'Added) 31 31 9.5 8 400 C 12 '1 11 1.2 0.6 350 O 2. 4 4 0. 5 0. 6 300-C 1. 5 1.6 0. 5 0, 6 270? G 1 0.9 0.5 0.6

Bismuth .With 1.5% ZjneParts PerMillion I na a Palladiunr Rhodium:Elnthecasewliere .5% zincwas used, .the total fission The results usingin the bismuth on addition of zinc.

Tellurium product contamination -was reduced from 182 parts per millionto less than three parts per million. In the case where 1.5 zinc wasused, the total fission product contamination was reduced from 511 partsper million at 500 C. to less than 4.5 parts per million at atemperature below 300 C.

In similar experiments with bismuth solutions of th individual fissionproducts, we have separated each of those products using as little as 2%zinc. In these experiments no detectable quantity of the fission productremained in the bismuth.

The temperature at which the zinc is added to and separated from thebismuth melt is relatively unimportant insofar as removal of the fissionproducts is concerned. For convenience, the zinc should be added at atemperature at which it will most rapidly dissolve in the bismuth. Inmost cases we have found that there is little advantage in separatingthe fission products at temperatures lower than about 350 C. asvirtually all ofthe fission products, except zirconium, are extractedfrom the liquid bismuth phase at temperatures higher than about 350 C.

I EXAMPLE II V The purpose of this experiment was to determine whetherfission products could be preferentially separated from bismuthcontaining uranium dissolved therein, i.e., without removing theuranium. A quantity of bismuth was heated in a graphite crucible in air.Fission product elements and uranium in the amounts indicated in Table 2below were added to the bismuth to form a solution. At 500 C., 0.3% zincwas added and the mixture agitated to form a homogeneous composition. Itwas noted that a dross formed at the surface of the bismuth. Samples ofthe liquid bismuth phase were taken, as in Example I, at thetemperatures indicated in Table 2 andanalyzed for their fission productcontent. The results are summarized below:

.It is noted that, as in the previous examples, the fission productconcentration was reduced to a negligible value However, these resultsappeared to indicate that the uranium content in the bismuth wasconsiderably decreased on addition of zinc. Further experiments wereundertaken to ascertain whether the uranium was in fact being removed bythe zinc, as we suspected that the loss of uranium was the "result ofoxidation by the air to which the solution was exposed rather thanreaction with thezinc. V

' EXAMPLE In- The object of this experiment was to determine the effectof zinc on a solution of uranium in liquid bismuth when a non-oxidizingatmosphere was maintained.

Bismuth containing a known concentration of uranium was melted undervacuum at 500 C The resultant ple .of the filtrate was analyzed foruranium..

In ap'arallel experiment, essentially the same conditions were used but,in addition, 3% zinc was added 'to the solution ofuranium in bismuth.Each sample was filtered under an atmosphere of argon at the severaltemperatures indicated in Table 3, and the bismuth filtrates analyzedfor uranium. The results are summarized below.

By comparison with the experiment "using no zinc, it is apparent thatthe zinc has relatively little aflinity for uranium in a liquid bismuthphase as compared to the ability of zinc to remove fission products. Theexcessive loss of uranium from the bismuth phase in Example II wasevidently the result of the oxidation of the uranium by air to which thesample was continuously exposed. It is also apparent from a comparisonof the results of the experiments summarized in Tables 2 and 3 that thefission products can be selectively and virtually quantitatively removedfrom the liquid bismuth containing uranium dissolved therein byconducting the separation in a non-oxidizing atmosphere, i.e., one inwhich the uranium cannot react chemically, such as by oxidation.Separation in an argon atmosphere or under vacuum has been found to besatisfactory. A nitrogen atmosphere should be avoided since the uraniummay react to form uranium nitrides.

Oxidation of the uranium may also be prevented by incorporating in thebismuth solution small amounts of metal which forms oxides more stablethan uranium oxides and whose solubility in liquid bismuth is relativelyunatfected by addition of zinc. For example, addition of about 100 to300 parts per million magnesium to the bismuth melt can reduce theoxidation of uranium to a minimum.

The concentration of uranium in the bismuth will be essentiallyunaffected if the fission products are extracted in the range 350500 C.If the initial bismuth solution contains smaller amounts of uranium thanabout 500 parts per million, the temperature at which the fissionproducts can be preferentially removed. without affecting the uraniumconcentration, can be lower.

The solubility of uranium in bismuth should be taken into considerationin using this invention. Care should be taken to avoid exceeding thesolubility of uranium in bismuth in all steps of the process.

Effective separation to the fission products may take place withoutcooling the resultant melt. See, for example, Table 1B which shows thatvirtually all of the fission products are extracted from the liquidbismuth at 450 C. without cooling. In general, however, it is desirableto cool the bismuth-zinc melt to a temperature slightly above itsfreezing point. The advantage of cooling is that the efficiency ofextraction is improved.

In separating zirconium from a solution of uranium in bismuth, mosteffective separation is obtained by cooling to a temperature in therange 270-300 C. At temperatures above 300 C. we have found that theconcentration of uranium and zirconium in a solution of these metals inliquid bismuth is substantially unaffected by the addition of zinc.

While we do not wish to be bound by any theory or mechanism, it isbelieved that preferential extraction of the fission products occurs asthe result of the formation of high melting zinc intermetallic compoundsand that these intermetallic compounds are preferentially soluble in thezinc phase.

Since many embodiments might be made in the present invention and sincemany changes might be made in the embodiment described, it is to beunderstood that the foregoing description is to be interpreted asillustrative only and not in a limiting sense.

We claim:

1. A method of purifying a solution of uranium in liquid bismuthcontaining at least one metal selected from the group consisting ofselenium, tellurium, palladium, ruthenium, rhodium, niobium, andzirconium which comprises contacting and mixing said solution in aninert atmosphere with zinc to form a homogeneous melt, allowing a solidzinc phase to form and separating said zinc phase containing said metal.

2. A method of purifying a solution of uranium in liquid bismuthcontaining at least one metal selected from the group consisting ofselenium, tellurium, palladium, ruthenium, rhodium, niobium andzirconium which comprises contacting and mixing said solution in aninert atmosphere to form a homogeneous melt, cooling said composition toform a phase containing zinc and said metal and separating said phasefrom the liquid bismuth.

3. A method of purifying a solution of uranium in liquid bismuthcontaining a metal selected from the group consisting of selenium,tellurium, palladium, ruthenium, rhodium and niobium which comprisescontacting and mixing said solution with zinc in an inert atmosphere toform a homogeneous melt and, at a temperature in the range of about 300C. to 500 C., separating a phase containing zinc and said metal.

4. A method of purifying a solution of uranium in liquid bismuthcontaining a metal selected from the group consisting of selenium,tellurium, palladium, ruthenium, rhodium, and niobium which comprisescontacting and mixing said solution in an inert atmosphere with ahypoeutectic concentration of zinc and, at a temperature in the range ofabout 300 C. to 500 C., separating a phase containing zinc and saidmetal.

5. A method of purifying a solution of uranium in liquid bismuthcontaining a metal selected from the group consisting of selenium,tellurium, palladium, ruthenium, rhodium, and niobium which comprisescontacting and mixing said composition with .22.S% zinc by weightbismuth in an inert atmosphere at a temperature in the range BSD-500 C.and separating a phase containing zinc and said metal.

6. A method of purifying a solution of uranium and zirconium in liquidbismuth containing a metal selected from the group consisting ofselenium, tellurium, palladium, ruthenium, rhodium and niobium whichcomprises contacting and mixing said solution with zinc in an inertatmosphere to form a homogeneous melt and, at a temperature 'in therange of about 300 C. to 500 C., separating a phase containing zinc andsaid metal.

7. A method of purifying a solution of uranium in liquid bismuthcontaining zirconium which comprises contacting and mixing said solutionwith zinc in an inert atmosphere to form a homogeneous melt and, at atemperature in the range 270 C. to 300 C., separating a solid zinc phasecontaining said zirconium.

8. A method of purifying a solution of uranium in liquid bismuthcontaining zirconium which comprises contacting and mixing said solutionin an inert atmosphere with a hypo-eutectic concentration of zinc and,at a temperature in the range 270 to 300 C., separating a solid zincphase containing said zirconium.

References Cited in the file of this patent UNITED STATES PATENTS2,721,813 Holmberg Oct. 25, 1955 2,758,023 Bareis Aug. 7, 1956 2,771,357Wroughton Nov. 20, 1956 2,778,730 Spedding et a1. Jan. 22, 1957

1. A METHOD OF PURIFYING A SOLUTION OF URAMIUM IN LIQUID BISMUTHCONTAINING AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OFSELENIUM, TELLURIUM, PALLADIUM, RUTHENIUM, RHODIUM, NIOBIUM, ANDZORCONIUM WHICH COMPRISES CONTACTING AND MIXING SAID SOLUTIO IN AN INERTATMOSPHERE WITH ZINC TO FORM A HOMOGENEOUS MELT, ALLOWING A SOLID ZONCPHASE TO FORM A HOMOGENEOUS MELT, ALLOWING A SOLID ZINC PHASE TO FORMAND SEPARATING SAID ZINC PHSE CONTAINING SAID METAL.